The present invention relates to paper making machines and, more particularly, to an apparatus for controlling shrinkage of a fiber web, such as a paper or paperboard web, during a drying process, an associated method, and a fiber web produced according to the method.
There has been an ongoing need to control the shrinkage of a paper web in the machine direction as well as perpendicularly to the machine direction (here referred to as the transverse direction) which occurs during the drying of the paper web. The magnitude of the shrinkage is determined by, for example, the properties of the papermaking pulp in question and the design of the drying section in question.
It is important for the process steps following the drying process, and for the final paper product, that the paper web shrinks uniformly, if at all, over the machine width. An uneven shrinkage profile gives the paper web undesirable physical properties, which may vary over the width of the web.
Uneven shrinkage of a paper web during drying can cause, for example, a paper curl, i.e. that a sheet of paper from the dried paper web will tend to roll itself up to a cylindrical form about an imagined axis. A reduced shrinkage promotes a final sheet of paper having a higher stiffness, no curl, and a smoother surface, wherein such characteristics are desirable for most paper and paperboard grades. In instances where the shrinkage is too little, the paper may, however, become too stiff for its intended purpose. For paper grades where high stretch at break, high tensile energy absorption, or low module of elasticity are desirable, it may be favorable to permit controlled shrinkage of the web during drying. Examples of such paper grades include kraft sack paper and uncreped soft paper.
The shrinkage in both the machine direction (MD) and the transverse direction (TD) influences the properties of the final paper. With modern machines, shrinkage in the machine direction can be controlled relatively well, e.g. by means of accurate control of the roll speeds and web tension through the drying section. However, it is typically more difficult to control shrinkage in the transverse direction (TD).
A significant contributor to web shrinkage is free draw between the drying cylinders in a drying section. Accordingly, a number of different arrangements have been developed which strive to restrain the paper web throughout the entire drying section in different ways, i.e. also at the free draws between the drying cylinders.
U.S. Pat. No. 5,864,965, for instance, discloses a solution which comprises holding the web edges to a drying fabric with suction boxes positioned between the drying cylinders in a drying section in order to counteract the shrinkage of the web in the transverse direction. According to U.S. Pat. No. 5,864,965, a partial vacuum is maintained along the edge portions of the web across essentially the entire free draw, wherein the vacuum is applied from the side of the drying fabric, which is turned away from the web. As such, the web will be steadily sucked to the drying fabric with forces that are greater than the counteracting forces acting in the transverse direction, thereby counteracting web shrinkage. The partial vacuum is stated as being set within the range 0.1 to 0.8 bar, preferably between 0.2 to 0.5 bar.
Furthermore, U.S. Pat. No. 4,680,873 describes an arrangement and a method of controlling or adjusting shrinkage and/or stretching of a paper web transverse to the transport direction in a drying section of a paper making machine, in which the drying section has the shape of a cylinder dryer and/or a fan dryer. At a point in the drying process when the paper web has a dry solids content of at most 75%, the paper is subjected to outward forces, which are forced to act at the edge portions of the web. The forces are brought to act upon adjacent, parallel edge portions of the paper web and are directed away from each other. The magnitude of these forces are stated to be chosen so that they are at the most 50% lower than the shrinkage forces which momentarily occur on the paper web at the specified dry solids content.
U.S Pat. Nos. 5,553,392 and 5,647,141 describe an apparatus and a method, respectively, for drying a wet sheet material, such as a paper web. The apparatus is equipped with an appropriate number of drying cylinders, which operate in cooperation with a plurality of steam openings provided adjacent to the drying cylinders. The apparatus incorporates one or more gas permeable material belts arranged to cooperate with the drying cylinders for transporting the sheet material. The material belt or belts is/are then looped about the different drying cylinders for urging the sheet material against their outer circumferential surfaces and holding the sheet material during the drying operation. It is stated that the purposes of what is described in U.S. Pat. No. 5,553,392, among other things, are to be able to limit the shrinkage of the paper web, and to achieve a high speed for removal of moisture, which gives an efficient and energy saving drying process.
U.S Pat. No. 5,737,848 describes a drying section for a running paper web with consecutive drying groups, each having drying cylinders. Felts or wires support the running paper web and rest against the drying cylinders. Guide rolls guide the web and the felts/wires. The guide rolls are arranged so that the running web is guided in a meandering manner in the transfer region between two consecutive drying cylinders, each associated with different felts or wires. In U.S. Pat. No. 5,737,848 it is stated, among other things, that the guide rolls are positioned in such a manner and that the felts are supported by the guide rolls in such a manner, that the paper web can be supported substantially continuously through the transfer region. It is stated that the purpose of the arrangement described in U.S. Pat. No. 5,737,848, among other things, is to reduce vibrations and to keep undesired shrinkage of the paper web to a minimum.
Further, U.S. Pat. No. 5,787,603 describes a drying section in a machine for production of a paper web, wherein the drying section incorporates a plurality of heatable drying cylinders. The drying section incorporates a support belt, which by use of guide rolls, encloses a portion of the circumference of each of the drying cylinders against which the paper web is urged. According to U.S. Pat. No. 5,787,603, the drying section incorporates pressing devices which, via the support belt, press the paper web against the drying cylinders. The pressing devices can include press rolls which are urged against the support belt, e.g. by means of springs. It is stated that one of the purposes of the arrangement disclosed in U.S. Pat. No. 5,787,603 is to be able to reduce the tension in the support belt without impairing the drying contact between the drying cylinders and the paper web.
U.S. Pat. No. 5,921,000 describes a drying section which utilizes top-felted and bottom-felted drying sections, each of which incorporates a single steam-heated drying cylinder. The paper web to be dried is transferred between the drying cylinders by means of a vacuum roll transfer without any open draw. The wrap of the machine fabric over the drying cylinders is over 270 degrees, which is considered to give a high drying capacity per drying cylinder. As each drying cylinder is followed by another drying cylinder that dries the opposite side of the paper web, it is stated that it is possible to secure the uniformity of the drying. Each drying cylinder might have its own stretching means and guide means for the machine fabric. It is stated that the paper web can be restrained during about 96% of the period of its passage through the drying section.
However, the earlier known arrangements for controlling the shrinkage or other dimensional change of a paper or paperboard web during the drying operation have certain shortcomings. For example, earlier systems having belt-like members, extending through the drying section for exerting forces upon the edges of the web, may not be capable of delivering a sufficient, continuous holding force for holding the web rigidly through the entire drying section to thereby prevent or control the shrinkage of the web in transverse direction. In addition, in earlier known arrangements where the web is supported or to some extent is firmly secured by aid of suction rolls, machine fabrics, guide rolls, sometimes in cooperation with drying cylinders, may also be incapable of ensuring a sufficiently large, continuous holding force throughout the entire drying section. Further, such types of earlier known arrangements are not capable of controlling the width of the web.
In the field of transverse stretching of plastic films and foils, some machine configurations may include an arrangement of pressure rolls, which directly or via V-belts, hold the edges of the films or foils therebetween in draws between the stretching rolls. Such an arrangement is described in DE-B 11 11 811. An arrangement of the type described in DE-B 11 11 811, applied to paper or paperboard production, however, would not be capable of maintaining a sufficiently large, continuous holding force on a running, moist paper or paperboard web during passage through a drying section.
Thus, there exists a need for a drying section in a paper making machine capable of controlling shrinkage in a fiber web during the drying process.
The above and other needs are met by the present invention which, in one embodiment, provides a drying section for drying and controlling shrinkage of a moist fiber web, the web having a predetermined width and opposed transverse ends, in a paper making machine. Such a drying section comprises a plurality of drying section members configured to receive and forward the web in a machine direction. Each drying section member has opposed ends in a transverse direction. The drying section members further comprise a first drying section member and a second drying section member, with the second drying section member being separated from the first drying section member so as to define an intermediate zone therebetween. The drying section further comprises a primary belt set having a belt member wrapping about each of the opposed ends of each of the drying section members. Each belt member is configured in an endless loop and has a width less than half of the width of the web. The drying section also comprises a first web-contacting element opposing the primary belt set, at least substantially through the intermediate zone, such that the web is disposed therebetween. The first web-contacting element cooperates with the primary belt set so as to at least partially constrain the web about the opposed transverse ends thereof to thereby control at least transverse shrinkage of the web.
Another advantageous aspect of the present invention comprises a method of drying and controlling shrinkage of a moist fiber web, wherein the web has a predetermined width and opposed transverse ends, in a drying section of a paper making machine. First, the web is forwarded about a plurality of drying section members configured to receive and forward the web in a machine direction. Each drying section member has opposed ends in a transverse direction. The drying section members further comprise a first drying section member and a second drying section member, with the second drying section member being separated from the first drying section member so as to define an intermediate zone therebetween. Next, the web is at least partially constrained in at least the transverse direction, and at least substantially through the intermediate zone, with a first belt set disposed opposite to the web from a web-contacting element, wherein both the first belt set and the web-contacting element operably engaging the web. The first belt set comprises a belt member wrapping about each of the opposed ends of each of the drying section members, wherein each belt member is configured in an endless loop and has a width less than half of the width of the web. The first belt set cooperates with the web-contacting element to at least partially constrain the web about the opposed transverse ends thereof so as to control shrinkage of the web.
In addition, a fiber web produced according to a method such as described herein would exhibit increased stiffness due to the shrinkage control experienced during the drying process. Consequently, for example, less papermaking fibers may be required in order to produce the paper grades having the same stiffness than in corresponding paper grades produced according to a process where shrinkage is not controlled.
Thus, embodiments of a drying section in a paper making machine according to the present invention are capable of controlling shrinkage in a fiber web during the drying process and producing an advantageous product thereby.